Three-dimensional hierarchical porous carbon foams for supercapacitors

ABSTRACT

A method of fabricating porous carbon foam includes mixing equal masses of SiO 2  particle dispersion with a chitosan solution, dropwise adding a glutaraldehyde aqueous solution into the mixture and solidifying it in air forming a room temperature hydrogel, lyophilizing the hydrogel to form a sponge-like SiO 2 -embedded aerogel, carbonizing in a furnace the aerogel to form a SiO 2 -embedded carbon foam, soaking the embedded carbon foam in NaOH to dissolve the SiO 2  particles to form a carbon foam having carbon sheets with sub-micron cavities, immersing the carbon sheets in de-ionized water to remove any NaOH residuals followed by drying, placing the carbon foam in KOH solution followed by drying, annealing in nitrogen atmosphere the dried carbon foam to synthesize a carbon foam with a multi-dimensional porous system, immersing the synthesized carbon foam in de-ionized water to prevent self-burning in air, and rinsing the carbon foam in HCl and water, then oven drying.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/082329 filed Sep. 5, 2018, which is incorporated herein by reference.U.S. patent application Ser. No. 16/082329 is a 371 of PCT applicationPCT/US2017/022491 filed on Mar. 15, 2017. PCT applicationPCT/US2017/022491 claims the benefit of U.S. Provisional application62/309184 filed on Mar. 16, 2016.

FIELD OF THE INVENTION

The present invention relates generally to chitosan derived carbonmaterial. More particularly, the invention relates to a combination oftemplate method and chemical activation to synthesize chitosan derivedcarbon foams with hierarchical porous structure and surface area ofnearly 3000 m²/g.

BACKGROUND OF THE INVENTION

Enhancing charge storage capacity at ultrafast charging and dischargingrates for electrical storage is an important topic of contemporaryscientific research. High-capacity and low-cost energy storage systemsare needed for technologies that include consumer electronics, andsupercapacitors. The shortcomings of lithium-ion storage technologiesinclude high costs and limited power density that restrict theirapplications.

What is needed is a three-dimensional hierarchical porous carbon foamthat can maintain substantially higher capacitance at ultrafast chargeand discharge rates than other conventional carbon materials that areused as supercapacitor electrodes.

SUMMARY OF THE INVENTION

To address the needs in the art, a method of fabricatingchitosan-derived porous carbon foam is provided that includes mixing aSiO₂ particle dispersion with a chitosan solution, where the mass of theSiO₂ particle dispersion is equal to the mass of the chitosan polymer,dropwise adding a glutaraldehyde aqueous solution into the mixture ofSiO₂ particle dispersion and chitosan, solidifying in air, theglutaraldehyde aqueous solution and the mixture of SiO₂ particledispersion and chitosan forming a room temperature hydrogel,lyophilizing the room temperature hydrogel to form a sponge-likeSiO₂-embedded aerogel, carbonizing in a furnace the sponge-likeSiO₂-embedded aerogel to form a SiO₂-embedded carbon foam, soaking theSiO₂-embedded carbon foam in sodium hydroxide (NaOH) to dissolve theSiO₂ particles to form a carbon foam having carbon sheets withsub-micron cavities, immersing the carbon foam having carbon sheets withsub-micron cavities in de-ionized water to remove any NaOH residualsfollowed by drying, placing the carbon foam in potassium hydroxide (KOH)solution followed by drying, annealing in nitrogen atmosphere the driedcarbon foam to synthesize a carbon foam with a multi-dimensional poroussystem, immersing the annealed carbon foam with a multi-dimensionalporous system in de-ionized water to prevent self-burning in air, andrinsing the carbon foam with a multi-dimensional porous system inhydrochloric acid (HCl) and water, then oven drying.

According to one aspect of the invention, the carbon foam with amulti-dimensional and hierarchical porous system has pore sizes in arange from sub-nm to tens of μm.

In a further aspect of the invention, the carbon foam with amulti-dimensional porous system has a Brunauer-Emmett-Teller surfacearea of 2905 m² g⁻¹.

According to one aspect of the invention, the synthesis process of thecarbon foam with a multi-dimensional porous system does not involvetoxic hydrocarbons.

In yet another aspect of the invention, the SiO₂ particle dispersion isformed using tetraethyl orthosilicate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B show SEM images of the morphology of the as-prepared PCF,where (1A) shows the three-dimensional interconnected network of carbonsheets and (1B) shows a single piece of porous carbon sheet, withmacro-pores of diameter around 2 μm uniformly dispersed on the carbonsheet, according to one embodiment of the invention.

FIGS. 2A-2B shown pore size characterizations, where (2A) shows thenitrogen adsorption-desorption isotherm collected for PCF at the liquidnitrogen temperature (77 K), and (2B) show the pore size distributioncurve derived from Barrett-Joyner-Halenda theory using the isothermshown in (2A), according to one embodiment of the invention.

FIGS. 3A-3B show the morphology of as-prepared carbon foams withmultiscale pores (CF-MP), where (3A) show a three-dimensionalinterconnected network of carbon sheets and (3B) shows a single piece ofporous carbon sheet, where uniformly distributed macro-pores withdiameters around 200 nm were created after removal of silicananospheres, according to one embodiment of the invention.

FIGS. 4A-4B show pore size characterizations, where (4A) shows thenitrogen adsorption-desorption isotherm collected for CF-MP at theliquid nitrogen temperature (77 K), and (4B) shows the pore sizedistribution curve derived from Barrett-Joyner-Halenda theory using theisotherm shown in (4A), where the curve shows the presence ofmicro-pores and meso-pores in CF-MP, according to one embodiment of theinvention.

FIG. 5 shows a flow diagram of the fabrication method for the ultrahighsurface area CF-MP, according to one embodiment of the invention.

DETAILED DESCRIPTION

The current invention provides a method to synthesize three-dimensionalhierarchical porous carbon foam (PFC) with inherent nitrogen-doping andextremely large surface area (2905 m² g⁻¹, measured byBruanauer-Emmett-Teller test). The hierarchical porous carbon foamelectrode delivers an outstanding specific capacitance of ˜240 F g⁻¹ atan ultrahigh current density of 500 A g⁻¹.

In one embodiment, a combination of using a template method and achemical modification to increase the surface area of chitosan derivedcarbon material is provided. The as-prepared carbon material achieves aremarkable surface area of ˜3000 m² g⁻¹, which is the highest valuereported for chitosan derived carbon materials.

In another aspect of the invention, the carbon structure is implementedas an electrode for a supercapacitor device. According to oneembodiment, the invention achieves outstanding specific capacitances atultrafast charging rates (e.g., ˜240 F/g at ultrahigh current density of500 A/g). These values are significantly better than the values obtainedfrom any other chitosan derived carbon electrodes under same measurementconditions. These values are also better than the values of most (if notall) of the carbon materials measured at ultrahigh charging rates.

The invention is capable of providing a high capacitance at ultrafastcharging and discharging rates. The three-dimensional hierarchicalporous foams can maintain substantially higher capacitance at ultrafastcharge and discharge rates than other conventional carbon materials thatare used as supercapacitor electrodes.

The current invention uses chitosan as the major starting material.Chitosan is the second most abundant natural bio-polymer that is readilyavailable in shrimp shells. The availability and sustainability ofchitosan can greatly reduce the material cost of supercapacitors.

In general, a method of fabricating chitosan-derived porous carbon foamis provided that includes mixing a SiO₂ particle dispersion with achitosan solution, where the mass of the SiO₂ particle dispersion isequal to the mass of the chitosan polymer, dropwise adding aglutaraldehyde aqueous solution into the mixture of SiO₂ particledispersion and chitosan, solidifying in air, the glutaraldehyde aqueoussolution and the mixture of SiO₂ particle dispersion and chitosanforming a room temperature hydrogel, lyophilizing the room temperaturehydrogel to form a sponge-like SiO₂-embedded aerogel, carbonizing in afurnace the sponge-like SiO₂-embedded aerogel to form a SiO₂-embeddedcarbon foam, soaking the SiO₂-embedded carbon foam in sodium hydroxide(NaOH) to dissolve the SiO₂ particles to form a carbon foam havingcarbon sheets with sub-micron cavities, immersing the carbon foam havingcarbon sheets with sub-micron cavities in de-ionized water to remove anyNaOH residuals followed by drying, placing the carbon foam in potassiumhydroxide (KOH) solution followed by drying, annealing in nitrogenatmosphere the dried carbon foam to synthesize a carbon foam with amulti-dimensional porous system, immersing the annealed carbon foam witha multi-dimensional porous system in de-ionized water to preventself-burning in air, and rinsing the carbon foam with amulti-dimensional porous system in hydrochloric acid (HCl ) and water,then oven drying.

In one exemplary experiment, chemical constituents were used thatinclude chitosan (M.W. 100,000˜300,000), acetic acid (glacial, 99.9 wt%), potassium carbonate (K₂CO₃), and glutaraldehyde aqueous solution (50wt %). All chemicals were used without further purification.

According to one exemplary fabrication process, 1 mL of 10 wt % K₂CO₃aqueous solution was slowly added into 20 mL of 1 wt % chitosan solution(with 1 wt % acetic acid as solvent) under vigorous stirring. Then, 240μL of 25 wt % glutaraldehyde aqueous solution was added dropwise intothe solution. After adding glutaraldehyde solution, the mixed solutionwas immediately transferred into a plastic container (e.g., a petridish). The solution slowly solidified in the air and formed thehydrogel. The as-formed hydrogel was exposed in air for 1 day to age andthen was frozen in a freezer. The frozen K₂CO₃-embedded chitosanhydrogel was freeze-dried for 48 h to form the chitosan aerogel.Pyrolysis of the aerogel was carried out in a home-built tube furnace innitrogen atmosphere at 800° C. for 2 hours to obtain PCF. The PCF waswashed with 0.1 M hydrochloric acid solution and de-ionized water toremove any soluble residuals.

The scanning electron microscopy (SEM) images or FIGS. 1A-1B reveal thePCF possesses a three-dimensional porous structure. Macro-pores ofdiameter around 2 μm are uniformly dispersed on each carbon sheet.

The porous structure of PCF was characterized by a nitrogen adsorptionand desorption experiment. As shown in FIG. 2A, PCF exhibits combinedcharacteristics of type I and type IV isotherms. The steep increase ofadsorbed nitrogen volume at low relative pressure (P/P₀<0.1) indicatesthe dominance of micro-pores in PCF. The hysteresis observed at P/P₀>0.4proves the existence of meso-pores (FIG. 2B). The surface area and porevolumes of PCF are summarized in Table 1. It clearly shows the presenceof micro-pores and meso-pores in PCF.

TABLE 1 Textural properties of PCF Total BET Pore Micropore SurfaceVolume, Volume, Meso-pore Ratio of Area, S V_(total) V_(micro) Volume,V_(micro) to Sample (m²/g) (cm³/g) (cm³/g) V_(meso) (cm³/g) V_(meso) PCF1013.0 0.576 0.461 0.154 2.994

Turning now to the preparation of hierarchical porous carbon foams fromglutaraldehyde-crosslinked chitosan aerogel with silica spheres, inanother exemplary experiment, chemical constituents were used thatinclude chitosan (M.W. 100,000˜300,000), acetic acid (glacial, 99.9%),tetraethyl orthosilicate (TEOS), potassium hydroxide (KOH), hydrochloricacid (HCl ), absolute ethanol, and glutaraldehyde aqueous solution (50vol %). All chemicals were used without further purifications.

In this exemplary fabrication process, SiO₂ spheres were prepared usingthe modified Stober method. First, 100 mL of absolute ethanol, 15 mL ofde-ionized water, and 5 mL of ammonia aqueous solution (25 wt %) weremixed in a 250-mL round-bottom flask. The mixture was stirred at 500 rpmand heated to 40° C. in a water bath. Second, 6.0 g of TEOS was quicklyadded into the mixture and kept the temperature at 40° C. for 6 h to letTEOS react with ammonia. After the reaction, the solution turned fromclear to white. The white precipitation (SiO₂ spheres) was collected bycentrifugation and rinsed with denatured alcohol (95 vol %) three timesfollowed by de-ionized water until the pH value reached 7. The cleanedprecipitation was re-dispersed in 20 mL de-ionized water prior to use.

In the preparation of silica embedded carbon foam (CF-SiO₂), SiO₂particle dispersion was slowly added into 18 g of 1.2 wt % chitosansolution (with 1 wt % acetic acid as solvent) under vigorous magneticstirring. The mass of added SiO₂ particles was equal to the mass ofchitosan powder. Then, 216 μL of 50 wt % glutaraldehyde aqueous solutionwas added into the mixture dropwise. Subsequently, the mixture waspoured into a container, e.g., a petri dish. The solution was graduallysolidified in air and formed hydrogel under room temperature. Theas-formed hydrogel was aged at room temperature for one day and thenlyophilized to obtain the spongy SiO₂-embedded aerogel. This aerogel wasthen pyrolyzed in a tube furnace at 800° C. for 2 hours in nitrogenatmosphere to obtain the CF-SiO₂.

Turning now to the preparation of carbon foam with templated walls(CF-TW), a piece of CF-SiO₂ was soaked into 2 M sodium hydroxide (NaOH)aqueous solution at 90° C. for 10 hours to completely dissolve the SiO₂particles. The obtained CF-TW was immersed in de-ionized water overnightto remove any NaOH residuals followed by drying in an electric oven at80° C. for 2 h.

For the preparation of carbon foam with multi-dimensional porous system(CF-MP), the aforementioned CF-TW was further reacted with potassiumhydroxide (KOH) to create micro-pores. First, the CF-TW was put in 1.0 MKOH solution for 8 h and then dried in an electric oven at 80° C. for 2hours. Second, the dried carbon foam was annealed in a tube furnace at800° C. in nitrogen atmosphere for 1 hour to synthesize the CF-MP. Afterthe annealing, the sample was quickly slid into a beaker of de-ionizedwater to prevent self-burning in the air. Finally, the obtained CF-MPwas rinsed by 0.5 M HCl and ample amount of water to pH˜7 and dried inthe oven at 80° C. for 2 h.

For the characterization of the CF-MP, FIGS. 3A-3B show the morphologyof as-prepared CF-MP. In FIG. 3A, the three-dimensional interconnectednetwork of carbon sheets is shown, and in FIG. 3B, a single piece ofporous carbon sheet is shown. Here, uniformly distributed pores withdiameters around 200 nm were created after removal of silicananospheres.

FIG. 4A shows the nitrogen adsorption-desorption isotherms of CF-MP.CF-MP exhibits combined characteristics of type I and type IV isothermswith steep increase of absorbed N₂ at low relative pressure (P/P₀<0.1)and slightly steep adsorption at P/P₀>0.8, indicating the co-existenceof micro-, meso-, and macropores. The textural properties of CF-MP aresummarized in Table 2. The CF-MP has ultrahigh surface area of 2905.6m²/g, much higher than conventional materials.

FIG. 5 shows a flow diagram of the fabrication method for the ultrahighsurface area CF-MP, according to one embodiment of the invention.

TABLE 2 Textural properties of CF-MP BET Total pore Micropore surfacevolume, volume, Ratio of area, S V_(total) ^(a) V_(micro) ^(b)V_(non-micro) ^(c) V_(micro) to Sample (m² · g) (cm³ · g) (cm³ · g) (cm³· g) V_(non-micro) CF-MP 2905.6 4.08 0.930 3.407 0.273

The present invention has now been described in accordance with severalexemplary embodiments, which are intended to be illustrative in allaspects, rather than restrictive. Thus, the present invention is capableof many variations in detailed implementation, which may be derived fromthe description contained herein by a person of ordinary skill in theart. For example, developing 3D printed hierarchical porous carbon foamelectrodes for supercapacitors and other devices, including batteries,catalysts and fuel cells.

What is claimed: 1) A method of fabricating three-dimensional porouscarbon foam for supercapacitors, batteries, catalysts or fuel cells,wherein the method comprising; a) mixing a SiO₂ particle dispersion witha chitosan solution; b) adding a glutaraldehyde aqueous solution intothe mixture of SiO₂ particle dispersion and chitosan; and c) forming thethree-dimensional porous carbon foam. 2) The method as set forth inclaim 1, wherein three-dimensional porous carbon foam has pore sizes ina range from sub-nm to tens of μm. 3) The method as set forth in claim1, wherein the three-dimensional porous carbon foam has aBrunauer-Emmett-Teller surface area of 2905 m² g⁻¹. 4) The method as setforth in claim 1, wherein three-dimensional porous carbon foam does notinvolve toxic hydrocarbons. 5) The method as set forth in claim 1,wherein the SiO₂ particle dispersion is formed using tetraethylorthosilicate. 6) The method as set forth in claim 1, wherein the massof the SiO₂ particle dispersion is equal to the mass of the chitosanpolymer. 7) A three-dimensional porous carbon foam for supercapacitors,batteries, catalysts or fuel cells, wherein the three-dimensional porouscarbon foam comprising; a) a mixture of SiO₂ particles and chitosan; andb) glutaraldehyde solidified with the mixture of SiO₂ particles andchitosan.